I have an ob1 that was turned down, I bought it from Bill at cornerstone. I think it's right around 12mm I will measure it tonight when I get home and let you.
I may even want to sell it if you are interested. It's a 5/16X14. I have two and just don't use this one that much anymore I am now just used to the 12.75mm. I did play with for about 3 months and I had no issues with it at all